(1) Field of the Invention
This invention relates to a microbubble positioning and control system, and more particularly, to a system for controlling the positioning of microbubbles near a Vessel's surface in water, using acoustic signals.
(2) Brief Description of the Prior Art
Ocean vessels and submarines encounter a great amount of drag when passing through water. The main cause of this drag in water is boundary layer turbulence at the wet surface of the vessel. The boundary layer is extremely thin. As the speed of the vessel increases, the boundary layer turbulence increases rapidly thereby increasing drag and preventing higher sustained speeds.
One well known technique for reducing drag is through gas bubbles injection at a porous surface into the boundary layer near the bow of a vessel. Bubbles, especially microbubbles with extremely small diameters, greatly reduce drag at the boundary layer, thereby increasing the speed and efficiency of the vessel. Microbubbles have been used for almost two decades in reducing drag on ocean vehicles. However, producing and maintaining a sufficient quantity of properly sized and positioned microbubbles in the boundary layer has been problematic.
The first problem is positioning the microbubbles within the boundary layer between the wetted surface of the vessel and the open water where the microbubbles can decrease turbulence and therefore reduce drag. In a system where gas is ejected through a porous surface into the water, a large portion of the bubbles are not positioned properly in the boundary layer and are wasted. Again, this is problematic if the vehicle is a submarine because of the volume of gas which must be stored. A system that positions the microbubbles into the boundary layer would greatly reduce the drag on the vehicle, without requiring a large amount of gas.
U.S. Pat. No. 4,398,925 discloses a method for positioning and moving bubbles in a liquid using acoustic energy. Acoustic energy is introduced into the liquid using a transducer. The acoustic energy reflects off an opposite surface of a container holding the liquid and reflects back to the acoustic transducer. A frequency is selected which creates a standing wave within the liquid. This acoustic standing wave has one or more antinodes, depending upon the frequency of the acoustic energy and the length of the container.
Antinodes in standing waves are places where the transmitted acoustic wave and the reflected acoustic wave cancel each other out. Bubbles will collect at these low pressure antinodes. Therefore, the '925 system discloses a method of positioning bubbles within a liquid volume using acoustic waves. Further, the '925 invention discloses changing the frequency harmonic of the transmitted acoustic signal, to create different antinodes to move the bubbles throughout the liquid solution.
This technique, however, will not work for a vessel in open water. U.S. Pat. No. 4,398,925 and U.S. Pat. No. 4,759,775 require a reflecting surface at a known distance to reflect the acoustic signal back to the source and create a standing wave field between the reflecting surface and the transmitting source. An ocean vessel, such as a ship or submarine, has no second reflective surface to reflect the sound waves.
Another problem is producing microbubbles of a preferred size, preferably on the order of 50 microns in diameter or less. The prior art approach is to eject gas through a porous surface into the boundary layer of an underwater surface. U.S. Pat. No. 5,117,882, for example, describes a microbubble generating and dispensing device utilizing felted metal fibers which are sintered to produce a porous surface for extruding a pressurized gas in the form of microbubbles into the boundary layer and using ultrasonic waves to help size the bubbles as they are produced.
However, no control is exercised over the size, position or distribution of the bubbles after they leave the porous surface. In order to maintain an efficient reduction in drag, a large amount of gas must be ejected through such a porous surface to maintain a preferred quantity of microbubbles in the boundary layer. The shortcoming of this drag reduction method is the large volume of gas which must be ejected, and hence carried on an underwater vehicle, to produce a significant level of drag reduction.
Accordingly, what is needed is a way of sizing bubbles near and at a boundary layer to produce a greater proportion of properly sized microbubbles suitable for reducing drag without using a large volume of gas to generate the bubbles.
Therefore, what is needed is a system for sizing and positioning microbubbles in the boundary layer proximate the outer surface of a vehicle. This will greatly reduce drag on the surface, while using a minimal amount of gas for creating the microbubbles.